This invention relates to a method of and an apparatus for controlling ignition timing of an internal-combustion engine, wherein the ignition is controlled to a predetermined degree of knocking by detecting a knock through the vibration or sound which is produced within and outside cylinders by the pressure in the cylinders of the internal-combustion engine.
Ignition timing of an internal-combustion engine has to be determined in accordance with the condition of the engine so that it can be operated in an optimum condition. In the conventional ignition timing control device, the ignition is normally determined in accordance with the engine condition represented by the engine speed and negative intake pressure which are detected by a centrifugal advancing mechanism and a vacuum advancing mechanism, respectively.
Taking into consideration the efficiency and fuel comsumption of a system, it is generally known that ignition is effected most effectively in the vicinity of MBT (Minimum advance for Best Torque). To this end, ignition time should be varied according to the engine condition in a manner that it is effected at MBT.
In a particular condition, however, a knock occurs as ignition time is advanced, preventing the engine from operating stably. In a low speed and low load situation, a knocking limit comes generally before MBT in the relation between MBT and the ignition causing a knock. The knocking limit is also affected by the ambient conditions such as temperatures and humidity. In order to prevent knocking under all operating conditions, commonly used ignition control devices are programmed by using parameters of the engine speed and negative intake pressure in a manner that ignition is effected substantially later than MBT. This has prevented the output and fuel consumption of the engine from being improved.
As well known, ignition is strongly correlated with the pressure within cylinders. When fuel/air mixture is ignited, the pressure within the cylinders does not carry harmonic components (normally 5 KHz to 10 KHz) if no knock occurs, but is affected by harmonic components if a knock occurs.
The effect of harmonic components is delivered to the outside of the cylinders in the form of vibration or sound. Various studies on so-called knocking feedback ignition systems have been made in which ignition is controlled by detecting this vibration or sound.
In such conventional ignition systems, knock detecting is performed during each combustion for every cylinder, and the detected signal is utilized, for example, to produce a certain lag if a knock exists and to produce a certain advance if no knock exists. Thus, ignition timing control is performed based on each combustion phenomenon.
In case of a multi-cylinder engine, great variations of combustion exist between the cylinders, because there are variations in fuel distribution and mixture distribution in the intake system and because individual conditions (deposit, compression ratio, etc.) in the combustion chambers are different. Therefore, if feedback control is performed by such an ignition system as described above, the hunting of ignition time becomes large and this disadvantageously causes surging and slackening.